The question is "how to label a vertice" and it's more complicated to label an angle !
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Alain MatthesJun 15 '11 at 13:56

@Altermundus: So something like tpub.com/math1/20.htm57.gif, especially the arrows that point to the start/end of the angle, is not easy doable?
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FlowJun 15 '11 at 16:28

Yes it's possible but not with the same facility ! :)
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Alain MatthesJun 15 '11 at 16:34

@Altermundus: Could you maybe elaborate this a bite more? I am new to tikz.
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FlowJun 15 '11 at 16:56

@Flow Without the arrows, it's simple. You can clip the triangle and draw a circle with center A. If you know the angle, it's also simple, you can draw an arc (look at my question tex.stackexchange.com/questions/20833/…) but If you don't know the value of the angle, you need to calculate this value and then you need to draw the arc.
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Alain MatthesJun 15 '11 at 18:20

Solution 3:
the solution in the accepted answer might be complicated for a beginner. And I tried the code, it seems the direction of the arc is not what I expected.

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My solution:
Of course there are many other good solutions. Regarding the problem I am interested in, I came up with a simple solution. I believe many people already know about this. Just post it here for a memo.

You are missing the angle label. Also the problems that you have given are also solvable but still this is also nice.
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percusseOct 5 '12 at 16:22

@percusse: I have added the label. (I don't know how to integrate the label and arc under one \draw command.) BTW, thanks for editing my answer. Based on your version, I revised it a little bit. I still prefer to introduce \p1,\p2,\p3 instead of only \p1 and \p2 because if I have other three points, I can simply put them in the p1,p2,p3 places, and need not to modify any other things.
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ShiyuOct 6 '12 at 3:35